A printing apparatus such as a printer, copying machine, facsimile, or the like print an image defined by dot patterns on a print medium such as a paper sheet, plastic thin plate, or the like on the basis of image information. Such printing apparatus can be categorized into an ink-jet system, wire-dot system, thermal system, laser beam system, and the like in accordance with the printing systems. Of these systems, the ink-jet system (ink-jet printing apparatus) prints an image by ejecting ink (print liquid) droplets from ejection ports of a print head and attaching them on a print medium. In recent years, a large number of printing apparatuses are used, and are required to meet high-speed printing, high resolution, high image quality, low noise, and the like. The ink-jet printing apparatus can meet such requirements. The ink-jet printing apparatus can stably print images on various print media since it can achieve a non-contact print process by ejecting ink from a print head.
As the ink-jet printing apparatus, a serial printing apparatus which prints an image by serially scanning a print head with respect to a print medium is prevalent, since it is difficult to increase the size of the print head. In such serial printing apparatus, various proposals have been conventionally made to improve the image quality of a print image.
For example, control that suppresses the influences of performance variations of respective print elements in a print head, and obscures the boundaries of print bands formed every time the print head scans, by printing a single raster in a plurality of scans is executed (to be referred to as “multi-pass print” hereinafter). In the multi-pass print process, it is a common practice to feed a paper sheet by a predetermined amount every time the print head is scanned once, in place of repeating head scans without feeding a paper sheet at all. For example, in case of a multi-pass print process using six passes, a nozzle array arranged in the head is divided into six in the paper feed direction, and a paper sheet is fed for each scan by a distance ⅙ the nozzle array length.
As a method for obtaining grayscale by two values “print/not print”, a method of defining grayscale by printing ink droplets corresponding in number to a required density to overlap each other within a grid (so-called “pixel”) of a given resolution (to be referred to as a dot overlap method hereinafter), a method of obtaining grayscale by setting an output resolution to be higher than an input resolution, and using a dot layout pattern corresponding to an input signal level (to be referred to as an index layout print method hereinafter), and the like are available.
Furthermore, the serial printing apparatus executes two-way print processes that print an image in reciprocal scans. Note that one of reciprocal directions is called a “forward” direction, and the other direction is called a “backward” direction in this specification.
However, in case of the two-way print processes, dot landing positions readily deviate in the forward and backward print processes, and such deviation of ink landing positions results in adverse effects such as granularity, color nonuniformity, and the like on a print image. In order to prevent such adverse effects, various proposals have been made.
For example, Japanese Patent Laid-Open No. 07-081190 has proposed a test print method, which can attain accurate register adjustment in the forward and backward directions by checking the uniformity of a pattern printed in the forward and backward print processes, on the basis of a method which is conventionally used as a register adjustment method and corrects dot landing positions based on vertical ruled lines printed in the forward and backward directions. Note that register adjustment means adjustment of the print position.
Moreover, Japanese Patent Laid-Open No. 2000-225718 has proposed a printing apparatus which prevents deterioration of image quality due to two-way deviations by nearly matching the barycentric position of a pixel with those of successive dots printed for that pixel upon forming one pixel by successively recording a plurality of dots.
However, such conventional multi-pass printing methods require complicated arrangements, since a process for selecting a dot layout pattern to be printed for each pixel and assigning dots of the selected pattern to a plurality of scans using a decimation mask is executed.
When the multi-pass print process is combined with the two-way print process, very precise positional deviation adjustment means between the forward and backward print processes is required. Furthermore, when the user starts a print process without adjusting any positional deviations, or when positional deviations recur due to aging even after they are adjusted, the adverse influences on an image are enhanced.